1. Field of the Invention
This invention relates to relatively high molecular weight, high density ethylene polymers capable of being formed with good processability into films having improved mechanical properties.
2. Description of Related Art
The following information is disclosed in accordance with the terms of 37 CFR 1.56, 1.97 and 1.98.
U.S. Pat. No. 4,307,209, issued Dec. 22, 1981, to Morita et al., discloses a process for producing bimodal ethylene polymers in two slurry stages in the presence of a catalyst comprising titanium, magnesium and halogen, with the polymer of the first stage being different in intrinsic viscosity and alpha-olefin comonomer content, and with the second stage carried out in the presence of the polymer of the first stage.
U.S. Pat. No. 4,352,915, issued Oct. 5, 1982, to Mashita et al., discloses a process for producing bimodal ethylene homopolymers or ethylene-alpha-olefin copolymers in two slurry stages in the presence of a catalyst comprising a titanium and/or vanadium compound supported on a specified magnesium compound, and wherein a relatively low molecular weight component is produced in the first stage and a relatively high molecular weight component is produced in the second stage.
U.S. Pat. No. 4,414,369, issued Nov. 8, 1983, to Kuroda et al., discloses a process for the production of bimodal polyolefins wherein olefin monomers are polymerized in a first stage in the presence of a Ziegler catalyst to produce a relatively high molecular weight polymer which is transferred to a second stage wherein further polymerization to a relatively low molecular weight polymer is effected, resulting in a final polymer having a wide molecular weight distribution.
U.S. Pat. Nos. 4,461,873, issued Jul. 24, 1984, and 4,547,551, issued Oct. 15, 1985, both to F. W. Bailey et al., each discloses blends of high molecular weight and low molecular weights ethylene polymers of purportedly narrow molecular weight distribution, useful in the production of films and blow-molded articles.
U.S. Pat. No. 4,048,412, issued Sep. 13, 1977, to Caumartin et al., discloses a process for the polymerization of olefins, e.g., ethylene, in a series of reaction vessels each operating in the gas phase and containing a fluidized bed of polymer and catalyst comprising a transition metal and organometallic compound as cocatalyst introduced into the first reactor, and wherein an additional amount of cocatalyst is introduced into a reactor other than the first. The patent discloses the use of different organoaluminum compounds as cocatalysts in the reactors for the purpose of varying the molecular weight distribution of the polymer.
U.S. Pat. No. 4,338,424, issued Jul. 6, 1982, to Morita et al., discloses a process for polymerizing olefins utilizing two gas phase polymerization zones, the first employing a higher hydrogen to olefin mole ratio to produce a low molecular weight (LMW) polymer and the second employing a lower hydrogen to olefin mole ratio to produce a high molecular weight (HMW) polymer. The process also includes a dilution zone between the two polymerization zones to which a fresh supply of olefin gas is added for feeding into the second polymerization zone.
U.S. Pat. No. 4,390,669, issued Jun. 28, 1983, to Morita et al., teaches the production of polyolefins utilizing two gas phase polymerization zones with the polymer from the first zone being fed to a suspension zone wherein it is suspended in a liquid hydrocarbon medium and the liquid suspension is fed to the second zone. A LMW polymer is produced in the first zone and a HMW polymer in the second zone.
U.S. Pat. No. 4,420,592, issued Dec. 13, 1983, to Kato et al., discloses the polymerization of olefins in the gas phase in a multiplicity of polymerization zones wherein a gaseous stream containing polymer from the first zone is fed to the second zone through a transfer passage containing an inert gas zone in which part of the gas components from the first zone is replaced by an inert gas. The disclosure is limited to the production of LMW polymer in the first zone and HMW polymer in the second zone.
U.S. Pat. No. 4,703,094, issued Oct. 27, 1987, to Raufast, discloses the production of polyolefins by the polymerization of alpha-olefins in the gas phase in several reactors, at least two of which are interconnected by a transfer device in which the gas mixture from the upstream reactor is subjected to decompression and compression stages, providing for elimination of the heat of reaction and degassing of the polymer powder.
U.S. Pat. No. 4,481,301, issued Nov. 6, 1984, to Nowlin et al., discloses catalysts for polymerizing alpha-olefins prepared by treating a support with an organomagnesium compound and contacting the supported magnesium composition in a liquid medium with a tetravalent titanium compound.
U.S. Pat. No. 4,888,318, issued Dec. 19, 1989, to Allen et al., discloses catalysts for the polymerization of alpha-olefins prepared by reacting a supported complex of titanium and magnesium with trimethyl aluminum.
European Published Patent Application No. 0 369 436, of Lee et al., published May 23, 1990, discloses a process for the production of ethylene copolymers in the gas phase utilizing at least two fluidized bed reactors in series under conditions such that a high melt index copolymer is made in one reactor and a low melt index copolymer in the other. The disclosure states that the high and low melt index polymers can be made in any order. The catalyst employed is a complex of magnesium, titanium, a halogen, and an electron donor on an appropriate support such as a silica or alumina, in combination with an organoaluminum activator and co-catalyst.
In accordance with this invention, relatively high molecular weight, high density ethylene polymers (HMW-HDPE) capable of being formed into thin films of high strength are provided, such polymers having a density of at least about 0.925 g/cc, a flow index (I21) no higher than about 15 g/10 min., a melt flow ratio (MFR) of at least about 65, and a dynamic elasticity (as defined hereinafter) at 0.1 rad./sec. of no higher than about 0.70 at a corresponding complex viscosity at 0.1 rad./sec. no higher than about 14E5, i.e., about 14xc3x97105 poises.
The ethylene polymer of this invention will in most cases consist essentially of a bimodal blend of relatively high molecular weight (HMW) and low molecular weight (LMW) ethylene polymers with the HMW component present, for example, at a weight fraction of at least about 0.5 of such ethylene polymer content and having a density of at least about 0.910 g/cc, a flow index (I21) of, for example, no higher than about 0.8 g/10 min., a relatively narrow molecular weight distribution indicated by a flow ratio (FR), for example, no higher than about 15, and a dynamic elasticity at 0.1 rad./sec., for example, of no higher than about 0.75 at a corresponding complex viscosity at 0.1 rad./sec. of no higher than about 7E6, i.e., 7xc3x97106 poises, and the LMW component having a density of at least about 0.940 g/cc and a melt index (I2) of at least about 100 g/10 min.
The HMW-HDPE product of this invention having relatively low elasticity and molecular weight distribution indicated by MFR, can be formed with good processability into thin gauge films having excellent mechanical properties, e.g., Dart Drop Impact and Elmendorf Tear resistance, despite the fact that its short chain branching, e.g., ethyl groups per 1000 C atoms, appears to be higher in the LMW component than in the HMW component. This is contrary to a prevailing belief that short chain branching should be concentrated in the HMW component for optimum mechanical properties of films.